This invention relates to a light detector such as a photoelectric switch and a light detecting IC used therefor.
Photoelectric switches of a type using an IC loaded with a light receiving element and a processing circuit therefor have been known. Such an IC will be hereinafter referred to as light detecting IC. FIG. 11 shows the structure of a conventional photoelectric switch of the so-called reflection type using a light detector 75 having a printed circuit board 74 loaded with a light emitter 73 provided with an LED 72 and a light detecting IC 70. Numeral 76 indicates a casing which constitutes the main body of the photoelectric switch. Numerals 77 and 78 indicate lenses respectively for projecting and receiving light and positioned respectively opposite the LED 72 and a photodiode 71 on the light detecting IC 70.
Light emitted from the LED 72 is projected forward through lens 77. If a target object to be detected is present in the travel path of this light, reflected light from a surface of the object is led through lens 78 to the light detecting IC 70. The light detecting IC 70 carries thereon not only the photodiode 71 for receiving the reflected light but also a processing circuit (not shown) for determining the presence or absence of a target object by processing the signal from this photodiode 71. The processing circuit makes this determination by comparing the level of the received signal with a specified threshold value and outputs a signal indicative of the result of the determination to an external device.
There have been other types of conventional photoelectric switches such as the so-called transmission type with a light emitter and a light receiver disposed opposite each other and the so-called optical fiber type using an optical fiber to transmit the light from the emitter or the reflected light from the target object.
Among the photoelectric switches of the reflection type, furthermore, there is also a so-called distance-setting type. A photoelectric switch of this type comprises a light detecting IC with a photodiode with two separated elements (hereinafter referred to as the divided photodiode) and capable of detecting the presence of a target object within a specified range of distance from the point of observation, for example, by considering the difference between the output signals from these photodiode elements and comparing the level of this differential signal with a specified threshold value.
In general, a photodiode with a large area must be used in a photoelectric switch if its detection distance (that is, the range of distance within which a target object is detectable) is to be set relatively long such that light from a large distance can be dependably received. Similarly, each element of a photoelectric switch of the distance-setting type must have a large area such that the difference in quantity of light received by the two elements of the divided photodiode can be accurately detected for a high-precision detection.
With a photoelectric switch of the optical-fiber type, it is also desirable to use a photodiode with a large area, especially when an optical fiber with a large diameter is used to transmit reflected light from a target object through an optical fiber to a light receiving element, such that divergent light propagating out of the end surface of the optical fiber facing the light receiving element can be dependably captured by the light receiving element.
If it is attempted to use a single light-detecting IC in common for a photoelectric switch with a long detection distance, a photoelectric switch of a distance-setting type, a photoelectric switch with an optical fiber having a large diameter so as to receive light efficiently and a photoelectric switch which does not require its detection distance to be too long, such that the effect of mass production will be improved, it becomes necessary to load a photodiode with a large diameter. Thus, the light detecting IC becomes large as a whole and its production cost is adversely affected. Even if the idea of using a single light-detecting IC is discarded and light-detecting ICs each loaded with a photodiode suited for a different kind of photoelectric switch are produced, the IC becomes large if loaded with a large photodiode or a divided photodiode. If an IC is produced such that the photodiode is to be connected externally, on the other hand, different ICs will have to be provided for different photoelectric switches because they have different processing circuits.
A brief survey reveals that most conventional photoelectric switches are of the transmission or reflection type, being used for purposes for which the detection distance need not be set too long and a photodiode with a modest area would be sufficient. Thus, it is cost-wise inefficient and increases the cost of an IC unnecessarily if light detecting IC loaded with a photodiode with a large area or a divided photodiode with two elements is produced for only infrequently used applications.
It is therefore an object of this invention to provide a cost-wise effective light detection IC having only one light receiving element with only a modest light receiving area but being connected to two separate light receiving elements or one large light receiving element such that output signals from such an externally connected element can be processed on the IC and hence that it can be used not only for situations where the detection distance need not be set long but also for situations where the detection distance is long, a distance is intended to be measured or an optical fiber with a large diameter is used.
It is another object of this invention to provide an inexpensive light detector of a type generating a signal for measuring distance by connecting such a light detecting IC with two separated light receiving elements and using output signals therefrom.
A light detecting IC embodying this invention, with which the above and other objects can be accomplished, may be characterized as comprising a single light receiving element, a first signal processing circuit for processing signals outputted from this light receiving element, an input terminal for receiving signals from outside, a second signal processing circuit for processing signals received from outside through the input terminal, and a switch circuit for selecting one or the other of these two signal processing circuits and outputting results of processing by the selected signal processing circuit.
The single light-receiving element is for receiving light from a light emitter disposed at a specified position or reflected from a target object of which the presence or absence is to be detected. A photodiode is usually used for this purpose. The first signal processing circuit serves to receive a signal from the light receiving element and to output the result of a signal processing carried out thereon according to the condition of the received light such as the result of comparison between the quantity of the received light and a specified threshold value. It may include an amplifier for amplifying the received signal, a comparator for comparing the amplified signal with the threshold value and an output circuit for outputting the signal received from the comparator to the exterior of the IC. The second signal processing circuit, like the first signal processing circuit, is structured so as to receive light from a light receiving element and to output a signal according to the condition of the received light. The switch circuit is for selectably connecting the single light receiving element with the first signal processing circuit or the input terminal with the second signal processing circuit. For making these connections, the switch circuit may include external terminals provided to the IC and its connecting lines. Suitable ones of these terminals are connected in order to select one of the signal processing circuits. The switch circuit may be installed on the IC. It may be placed anywhere with respect to the first and second signal processing circuits. It may be placed on the downstream side of the amplifier or of the comparator. It may be placed downstream to the first comparator and upstream to the second amplifier.
If the switch circuit selects the first signal processing circuit, the IC can be used for the ordinary purpose of a photoelectric switch (of either the transmission or reflection type) since the result of signal processing according to the condition of light received by the single light receiving element is outputted. If a light receiving element divided into two parts or a large light receiving element with a larger light receiving surface than that of the single light receiving element on the IC is connected to the external terminal and the switch circuit selects the second signal processing circuit, on the other hand, the result of processing in accordance with the condition of light received by the externally connected light receiving element is outputted. Thus, the IC can be used for a light detector of the distance-setting type or for a detection which would require a large light receiving area such as a detector using an optical fiber. Since light receiving elements of different kinds can be easily attached to the external terminals, an appropriate kind of light receiving element can be selectively attached to the external terminals, depending on the kind of light emitter used for the light detection.
In order to have a light receiving element of the type divided into two parts connected to the light detecting IC thus structured, the IC must be provided with a pair of external terminals. The second signal processing circuit is then structured with differential amplifiers for generating a differential signal representing the difference between the signals inputted through the external terminals and a comparator for comparing the level of this differential signal with a specified threshold value.
The light receiving elements of such a divided photodiode are individually connected to the external terminals. If these light receiving elements are placed at different positions with respect to the light emitter, a detection signal indicative of whether a target object is present within a specified range of distance from the point of observation can be outputted. If the differential signal is compared with a plurality of threshold values, it is possible to output a signal indicative of the distance or of the target object from the observation point.
If a light receiving element with a larger light receiving surface than that of the single light receiving element on the IC is connected, it is connected to one of the pair of the external terminals, the other terminal being grounded. In this case, the differential signal is indicative of the quantity of light received by this larger light receiving element. The differential signal thus obtained may now be processed (such as for comparing the signal level with a threshold value) by the first signal processing circuit.
According to a preferred embodiment of the invention, the first signal processing and the second signal processing circuit may be structured so as to share a single common circuit for the output of the result of processing. The circuit for outputting the result of signal processing is for outputting a final detection signal indicative of the presence of absence of a target object. If a circuit is required for matching the levels of the input signals through the internal light receiving element and the input signal through the external terminal, such a circuit may also be shared between the first and second signal processing circuits. When there is such a circuit connected in common to both the first and second signal processing circuits, the switch circuit serves to switch the connection between its common part and its individually dedicated parts.
A light detector embodying this invention is characterized as comprising such a light detecting IC provided with a pair of external terminals and a light detecting element divided into two parts connected to this pair of terminals. It is preferable to have both the IC and the divided light receiving element mounted to a single printed circuit board.